We did an Alaska cruise out of Seattle a few years ago; 2nd week of Sept. on Carnival Legend. Loved it! We really didn't think about the colors; thought it might be too early. But we were pleasantly surprised by the amount of color we saw. I think the most we saw were bright yellows of what I think were aspen trees. Lots of late wild flowers were blooming and the lichens/mosses had also turned to their fall colors. The main place were got to experience the fall foliage was our port day in Skagway. We rented one of the very few available cars and drove up the Klondike Hwy into Canada (must have passport) up to Emerald Lake. The highway, then, was in great condition and there were very few cars. Literally saw maybe 15-20 all day. We had lunch at a little cafe in the middle of nowhere near Emerald Lake, then headed back to Skagway. There is a train excursion there, but we chose the rental car to have control over when/where we stopped. Below are a few of my Alaska pics. I saw some from another cruiser who went there 1-2 weeks after we were there, and there was a huge increase in fall leaf color when they were there....so last half of Sept had more color. I'm sure like everywhere, the leaf color varies from year to year!
We are doing a Canada (St. John, Halifax, Sydney) 7-day cruise out of NYC in a few weeks. We're curious as to how much color we may see! Whatever you choose, just have fun and enjoy!